In many structures, panels have attached reinforcing structural members to improve buckling stability. In aircraft, a skin panel (e.g. a wing cover) may have reinforcing structural members (e.g. stringers). The reinforcing structural members meet with various obstructions and so need to be “run-out”, or terminated. The problem of transferring load between the reinforcing member and the skin panel that it is reinforcing in the run-out region is well known. A bond line is typically used to attach the stringer flange to the skin and may be used in conjunction with bolting to transfer load into the skin.
Conventional metallic stringers can be machined from a given cross section (e.g. ‘I’, ‘T’, ‘L’) to a flat extended flange at the termination. The stringer can have an initial web taper (i.e. of reducing web height) to facilitate load transfer by providing gradual decrease of transverse bending and axial stiffness. The attached flat extended flange renders the stringer tip more compliant with the skin when it is bent due to the eccentricity of the in-plane loads. The effect of lowering the neutral axis of the stringer closer to the skin and gradually reducing the stiffness of the stringer through significantly reducing the cross-sectional area, and second moment of area, at the run out, is to gradually transfer load through bonding and/or bolting.
Composite stringers usually comprise a back-to-back interface in the central web section but these cannot be run out in this ideal fashion, as a minimum attachment height must remain above the noodle (the fibre filler in the cleft at the interface) to prevent web cracking under load. This therefore limits the lowering of the neutral axis and cross-sectional area reduction. As a result, geometric tapering and hence load transfer in the stringer termination region, tends to be relatively abrupt with poor compliance with the skin under bending.
Bonding or co-curing of the reinforcing member to the skin aggravates this situation; due to high bond line stiffness and peak shear loads at the attached flange termination position. These bonded joints also tend to be susceptible to peeling failures. These factors contribute to cause premature cracking, peeling and disbond growth in these regions. This can be exacerbated in composite panels if there is significant stiffness mismatch between the reinforcement member and the skin. Such issues are important to overcome in order to realise potential weight saving benefits of co-curing/co-bonding structures as well.
A typical run-out for a composite ‘T’ stringer comprises an angled taper of the web section until the residual height is enough to ensure an adequate bonded area between back-to-back web sections. As this geometric tapering may not be sufficient to terminate a stringer at its desired location, another structural member (e.g. an aircraft wing rib) inboard of the desired termination location may be used to help transfer loads more effectively, delaying crack initiation/disbond. As a result of this earlier termination, the skin thickness must be increased following the stringer run-out giving rise to a weight penalty. Furthermore, additional supporting structural members may require modification to account for new loads, and repair of the run-out region becomes more complicated.
Further geometric modifications in the stringer termination region are possible but these have associated tooling, tolerance and manufacturing complexities. It is therefore desirable to make the composite stringer more compliant in the termination region though non-geometric modifications. Additionally, or alternatively, it is desirable to make a stringer more compliant at one or more locations in-board of the run-out region. More generally, it is desirable to make a region of any composite structural member more compliant through non-geometric modifications.